Fibre optic sensors are becoming a well-established technology for a range of applications, for example geophysical applications. Fibre optic sensors can take a variety of forms, and a commonly adopted form is to arrange a coil of fibre around a mandrel. Point sensors such as geophones or hydrophones can be made in this way, to detect acoustic and seismic data at a point, and large arrays of such point sensors can be multiplexed together using fibre optic connecting cables, to form an all fibre optic system. Passive multiplexing can be achieved entirely optically, and an advantage is that no electrical connections are required, which has great benefit in harsh environments where electrical equipment is easily damaged.
Fibre optic sensors have found application in downhole monitoring, and it is known to provide an array of geophones in or around a well to detect seismic signals with the aim of better understanding the local geological conditions and extraction process. A problem with such an approach is that geophones tend to be relatively large and so installation downhole is difficult. In addition geophones tend to have limited dynamic range.
WO 2005/033465 describes a system of downhole acoustic monitoring using a fibre having a number of periodic refractive index perturbations, for example Bragg gratings. Acoustic data is retrieved by portions of the fibre and used to monitor downhole conditions.
There are numerous different processes involved in formation and operation of a production well. Typically, to form a well, a borehole is drilled to the rock formation and lined with a casing. The outside of the casing may be filled with cement so as to prevent contamination of aquifers etc. when flow starts. Once the well bore has been drilled and lined the casing is typically perforated. Perforation involve firing a series of perforation charges, i.e. shaped charges, from within the casing that create perforations through the casing and cement that extend into the rock formation. Once perforation is complete, in some wells in is necessary to fracture the rock to provide a flow path for the oil/gas. Typically the rock is fractured in a hydraulic fracturing process by pumping a fluid, such as water, down the well under high pressure. This fluid is therefore forced into the perforations and, when sufficient pressure is reached, causes fracturing of the rock. A solid particulate, such as sand, is typically added to the fluid to lodge in the fractures that are formed and keep them open. Such a solid particulate is referred to as proppant. The well may be perforated in a series of sections, starting with the furthest section of well from the well head. Thus when a section of well has been perforated it may be blocked off by a blanking plug whilst the next section of well is perforated.
Once all perforations are complete the blanking plugs may be drilled out and production tubing installed. Sand screens and/or gravel packs may be placed to filter the in-flow and packers may be placed between the production tubing and the casing. In wells where the reservoir pressure is insufficient it may be necessary to install artificial lift mechanisms.
Once the well formation is completed production flow can be started.
During the formation of the well there are therefore many downhole processes that are conducted and generally very little information is available regarding what is happening down the well. Conditions at the top of the well can be monitored, such as flow rate of a material into or out of the well. Distance into a well bore may be determined by measuring deployment of a cable attached to a piece of apparatus. However it is generally very difficult to receive feedback from the location of the process itself. The well conditions are normally hostile and especially so when fracturing or perforation is taking place for instance. Further, even when the well is complete, there is a need for various tests and monitoring to take place, which often require halting production and deploying wire line logging tools.